Patent GB-947306-A (GB)
Title: Improvements in and relating to signal modulating apparatus
IPC: H04H20/88
Filed: 1960-02-12
Abstract: 947,306. Multiplex radio signalling. PHILCO CORPORATION. Feb. 12, 1960 [Feb. 12, 1959], No. 5050/60. Heading H4L. Relates to a system for transmitting stereophonically related signals in which a signal including a difference component of the two signals phase modulates a carrier wave and a signal including a sum component of the two signals amplitude modulates the phase-modulated carrier wave. The arrangement is particularly suitable for modifying an existing broadcasting station. As shown in Fig. 1, the existing monaural transmitter, below the broken line 10, comprises a carrier wave generator 12, limiter 14, amplifier 16, amplitude modulator 18, and amplifier 24. The connection between the oscillator 12 and limiter 14 is broken and the carrier wave supplied to a suppressed carrier modulator 88 and through a 90 degrees phase shifter to an adder 92. Microphones 30, 32 supply the two signals A, B to a matrix 34, Fig. 2 (not shown), which supplies the combined low frequency audio signals to each output 66, 68, and supplies the two high-frequency audio signals to the respective outputs. The output signals A<SP>1</SP>+B<SP>1</SP>, from the matrix 34 are added at 70 and subtracted at 72, and the output from 72 is squared at 76 and added at 74 to the output from 70. The signals from the matrix 34 are also supplied to square law amplifiers 82, 80, respectively, the amplifier 82 being followed by an inverter 84, and the signals from 72, 80 and 84 are added at 86. The carrier oscillations and the signals from the adder 86 are supplied to the suppressed carrier modulator 88 and the resultant signals combined with quadrature carrier oscillations at 92. The resulting phasemodulated carrier wave is amplitude modulated at 18 by the signal supplied by the adder 74. In a modification, Fig. 3 (not shown), the square law amplifier 76, adder 74, and limiter 14 are omitted and the output of the adder 70 is supplied directly to the amplitude modulator 18. In an alternative arrangement, Fig. 5, the A and B signals are supplied via the matrix 34 to amplitude modulators 130, 132, respectively, which receive quadrature carrier oscillations from a reference oscillator 134. An adder 138 combines the outputs of the modulators 130, 132, and the output of the adder is applied to an envelope detector 140 which supplies a signal to amplitude modulate the phasemodulated carrier at 18. The phase modulation of the carrier supplied by the oscillator 12 is obtained by limiting the signal from the adder 138 at 142 and supplying it to a discriminator 144 and integrator 146 to provide a signal proportional to the phase modulation present. A suppressed carrier modulator 88 receives the output of the integrator 146 together with carrier oscillations from 12 and the output of the modulator 88 is combined with quadrature carrier oscillations at 92. Vector diagrams are given to show that the modulation components are substantially the same as those produced by the apparatus shown in Fig. 1. In a further arrangement, Fig. 6, sum and difference signals from the matrix 34 are supplied to respective suppressed carrier modulators 150, 152, receiving carrier oscillations in quadrature derived by a mixer 154, and oscillator 156, from the main oscillator 12. The outputs of the two modulators are added at 162 together with a reduced carrier from the mixer 154. The signal from the adder 162, consisting of a carrier wave amplitude modulated by the A<SP>1</SP> + B<SP>1</SP> signal and phase modulated by the A<SP>1</SP> - B<SP>1</SP> signal, together with oscillations from the generator 156 are supplied to a mixer 166. The heterodyne sum signal from the mixer 166 is supplied via an adjustable delay circuit 172 and limiter 14 to the amplitude modulator 18 and is also supplied via an envelope detector 168, to the amplitude modulator.